<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .father {
            width: 100px;
            height: 100px;
            font-size: 30px;
            border: 1px solid red;
        }
        .son {
            width: 50%;
            height: 50%;
            font-size: 100px;
            border: 1px solid yellow;
            line-height: 50%;
            vertical-align: 50%;
        }
    </style>
</head>
<body>
<div class="father">1
    <div class="son">2</div>
</div>
</body>
</html>